The invention disclosed and claimed herein generally pertains to improved apparatus for connecting a high voltage (HV) electric cable to an X-ray tube. More particularly, the invention pertains to apparatus of the above type which effectively transfers heat through the connector apparatus, so that heat generated in the X-ray tube is not trapped in a region proximate to the connector. Even more particularly, the invention pertains to apparatus of the above type which employs an elongated heat transfer device, such as a heat pipe or the like, to enhance heat dissipation with respect to the connector apparatus.
In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum and across very high voltage, on the order of 100 kilovolts, from a cathode to a focal spot position on an anode. X-rays are produced as electrons strike the anode, comprising a tungsten target track, which is rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, typically less than 1% of the total power input. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, heat removal, or other effective procedure for managing heat, tends to be a major concern in X-ray tube design.
In a common arrangement, an HV electric power cable is employed to provide the requisite 100 kilovolt potential difference between the cathode and anode, in order to produce X-rays as stated above. One end of the cable is connected to a power source of sufficiently high voltage, and the other end is connected into the tube, for connection to the cathode, by means of an HV connector assembly. The connector assembly generally comprises structure for holding the end of the cable in place with respect to the tube, so that the end portion of the cable conductors, which may comprise either a single conductor or a number of conductors, can be joined to a tube. Accordingly, the connector assembly further comprises a quantity of HV insulation placed to surround any exposed portion of the cable conductors which lie outside the tube. The HV insulation is joined to the X-ray tube and is comparatively thick, in view of the high voltage of the cable conductors.
Generally, good high voltage insulating materials, such as epoxy, also tend to be very poor thermal conductors. This can create a very undesirable situation, if an HV connector assembly of the prior art is directly attached to an X-ray tube, such as across an end thereof. As stated above, a great deal of heat is generated in the X-ray tube, as an undesired byproduct of X-ray production. Some of this heat is directed against the connector insulation material, which has a comparatively large area in contact with the tube. Because of its poor thermal conductive properties, this insulator serves as a heat barrier, so that a substantial amount of heat tends to accumulate proximate to the connector. As a result, the temperature limits of the connector insulation may be readily exceeded, so that the steady state performance of the X-ray tube must be limited.
In one previous arrangement for dealing with this constraint, a reservoir of cooling oil is placed between the HV connector and structure inserted into the tube to support the cathode. However, this arrangement requires that the oil serve as a dielectric. In another arrangement, cooling oil is circulated through the HV connector. This arrangement, however, requires a completely separate oil circuit, provided with tubing and a circulation pump. Thus, this approach can significantly increase cost. In a third prior art arrangement, a good thermal conductor is placed in the electrical insulation of the HV connector to enhance heat flow. However, such thermal conductors can compromise or degrade dielectric characteristics, and have tended to diminish the electrical insulating capabilities of the HV connector assembly.
The invention provides apparatus for connecting a high voltage electric cable to an X-ray tube, wherein the apparatus may be attached directly to the tube, such as to the outer surface of the tube casing. The apparatus effectively insulates any exposed portions of the HV cable conductors, and at the same time readily dissipates heat from regions proximate or adjacent to the connector apparatus. The apparatus generally comprises a housing joined to the X-ray tube, and a quantity of selected electric insulating material contained within the housing, the insulating material being traversed by a portion of the HV cable. The apparatus further comprises an elongated heat transfer device positioned within the insulating material to extend along the traversing portion of the cable, in closely spaced relationship therewith. A quantity of selected working fluid is sealably contained in the heat transfer device, the working fluid being disposed for bidirectional movement along the device to transfer heat from a first location within the insulating material to a second location which is proximate to or outside of the housing. By placing the heat transfer device along the cable, and more particularly along the electric conductors thereof, the transfer device does not cut across voltage potential lines, and therefore will not interfere with the electrical insulating requirements of the HV connector.
Preferably, the heat transfer device comprises a conduit segment of selected length, the conduit segment having an inner wall in adjacent relationship with a sealed interior space. A selected porous material is attached to the inner wall and configured to define a passage through the sealed interior space that extends along the length of the conduit segment, the porous material being selected in relation to the working fluid so that the fluid, when in liquid form, is disposed for movement through the porous material by means of capillary action. When the first location is at a selectively higher temperature than the second location, fluid proximate to the first location is vaporized into gaseous form, moved along the passage by means of convection to the second location, and then condensed into liquid form.
In one useful embodiment, the conduit segment is placed or positioned with respect to the cable so that the electrical conductors of the cable extend through the center of the conduit segment, along the axis thereof. The conduit segment comprises a selected electrically conductive material. A sleeve, likewise formed of electrically conductive material, is positioned within the conduit segment, in coaxial relationship therewith, between the sealed interior space and the conductors of the cable.
In a second useful embodiment, the apparatus is provided with a sleeve of selected electrically conductive material which is placed around the cable conductors. The conduit segment comprises one of a plurality of substantially identical conduit segments which are positioned around the outer surface of the sleeve, in abutting relationship therewith and equally spaced apart from one another.